Change of state current limiter having flat plate construction



J. J. KEENAN 3,389,360

CHANGE OF STATE CURRENT LIMITER HAVING FLAT PLATECONSTRUGTION June 18, 1968 2 Sheets-Sheet 1 Filed April 1.9, 1967 lm enfar James J. Keenan by His Affor wy I June 18, 1968 J. J. KEENAN e CHANGE OF STATE CURRENT LIMITER HAVING FLAT PLATE CONSTRUGTIQN Filed April 19, 1967 2 Sheets-Sheet 2 I 72 40 44 WI/I/ X/ 70 lnvenfor: James J. Keenan His Ahomey.

United States Patent Olhce 3,389,360 Patented June 18, 1968 3,389,360 CHANGE OF STATE CURRENT LIMITER HAVING FLAT PLATE CONSTRUCTION James J. Keenan, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Filed Apr. 19, 1967, Ser. No. 632,050 8 Claims. (Cl. 337114) ABSTRACT OF THE DISCLOSURE tained in compression between a pair of headers which contain pistons and resilient means allowing pistons to recede under pressure.

The present invention relates to fault-currents limiting devices utilized in circuit interruption applications, and more particularly, relates to such devices which are selfhealing and adapted for multiple usage and in which the current limiting function is provided by the change of resistance of a conductor which undergoes a change of state from liquid to vapor upon the occurrence of a faultcurrent.

As power systems increase at all levels in size, power capability, and strength and degree of interconnections, the possibility of destructive fault-current occurrences and the diffieulty of protecting system components from the deleterious effects of destructively high fault-currents greatly increases. Such fault-currents represent a potential source of damage which requires circuit protection. In high voltage, high current applications, such protection may readily be obtained by the use of complicated and expensive circuit interrupters, circuit breakers and similar devices, the cost of which is readily justified by the inability to control the exceedingly high voltage and currents by any other means.

At lower voltage levels, however, as for example service lines and secondary lines wherein the voltages may range up to 1000 volts, the use of such complicated and expensive mechanisms is not commercially feasible. Such circuits are generally protected by a combination of a current limiting fuse and a relatively low power circuit breaker, as for example, a molded case type circuit breaker. In operation, when the fault-current occurs, the current limiting fuse lowers the current which is seen by the circuit breaker by a self-destructive dissipation of the transient peak thereby so that the relatively low current that remains may be interrupted by the low current breaker without the necessity of utilizing expensive circuit interrupting mechanisms.

Although this type of current limiting interruption is entirely feasible and economically attractive in many ap-' plications where it is not unduly burdensome to replace the current limiting fuse before it is necessary to prepare for return to use, it is desirable that devices be provided which are self-healing and which do not cause such replacement to be necessary before the circuit is rendered operative again. 7 7

Recently, reusable, non-destructive current limiting devices have been developed, as for example, those set forth in US. Patent No. 3,117,203, issued Jan. 7, 1964 to R. L. Hurtle. Such devices rely upon a change of state of a conductor which is liquid in the colder class and condition and vaporized in the high current or hot condition, thus causing a large increase in circuit resistance, which limits the current and allows for the interruption by a molded case type circuit breaker, for example. Such devices are useful for relatively low voltage and currents and are quite satisfactory in that respect. It is, however, desirable that the power handling capacity of non-destructive change of state current limiter devices be extended and that the useful life of such devices be rendered more nearly optimum. It is also desirable that the cost per interruption by such devices be decreased and that currents and voltages which may be interrupted by combination of such devices with low power cricuit interrupters may be greatly increased without a commensurate increase in cost.

In the copending, concurrently-filed application of L. P. Harris, application Ser. No. 632,049, filed Apr. 19, 1967, assigned to the assignee of the present invention, there is disclosed a high power, high current change-of-state current limiter device which accomplishes the foregoing objectives quite well. While the current limiter devices disclosed in the Harris application are a great improvement upon prior art change-of-state current limiter devices, it is required that numerous parts be fabricated to a very close tolerance in order that such devices be operative. It is desirable that the complexity of such devices be reduced and that the necessity for maintaining close tolerances of a large number of pieces be minimized.

Accordingly, it is an object of the present invention to provide change-of-state current limiter devices suitable for operation at high voltages and high currents and adapted to be fabricated with a minimum number of operating parts.

' Yet another object of the present invention is to provide non-destructive change-of-state current limiter devices which may be utilized at high voltages and currents which are capable of self-healing operations for a large number of current limiting operations and which are simple and economical to manufacture. In accord with the present invention I provide a changeof-state current limiter device in which a metallic, currentcarrying conductor having a relatively low resistance to the flow of electric current in the cold, or quiescent, state is operative, upon the occurrence of a fault-current having a magnitude of thousands of amperes, to become vaporized and thus present a high resistance to current flow which effectively limits the value of current by nondestructive dissipation within the device. In further accord with the invention, the channel, which contains the current carrying metallic conductor, is made by cutting matching grooves in the mating faces of a pair of ceramic members which are held together in compression with sufficient force to resist the thermal and mechanical shock attendant a transition of the current carrying metal from a liquid to a vapor state in the current limiting operation.

In further accord with the present invention, I provide a pair of high strength, high ductility steel headers for holding the channel-containing, ceramic members in compression which header contains a cavity which operates as a terminal point for the are that is established through the vaporized metallic current conductor during current limiting operation and contains metal which is readily wet by the current conductor to function as a footpoint for the arc. Also contained within the header is a reciprocable piston for allowing expansion of the volume within which the metallic current carrying conductor is contained to minimize mechanical and thermal shock when the conductor changes from the liquid to the vapor state. Each piston is supported resiliently by a compressible member to facilitate maintaining the piston in mechanical contact with the metallic current carrying conductor at all times prior, to and during current limiting.

The novel features characteristic of the present invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood with reference to the following detailed description taken in connection with the appended drawing in which:

FIGURE 1 is a vertical cross-sectional view, with parts broken away, illustrating a fiat plate current limiter device constructed in accord with the present invention,

FIGURE 2 is a plan view of the device of FIGURE 1,

FIGURE 3 illustrates a perspective view of one of the flat plates which defines a portion of the channel for containing the metallic current carrying conductor of the device of FIGURE 1,

FIGURE 4 is an expanded cross-sectional view of the member illustrated in perspective in FIGURE 3,

, FIGURE 5 is a vertical cross-sectional view of an alternative insert for the device of FIGURE 1 which may be utilized in lieu of the device illustrated in FIGURES 3 and 4, and

FIGURE 6 is a graphical representation of essential parameters useful in understanding the physical mechanism involved in the change-of-state utilized in current interrupters in accord with the present invention.

In FIGURE 1 of the drawing, limiter 10 comprises an upper header 11 and a lower header 12 connected together with a plurality of bolts 13 and holding, in compression therebetween, a channel-containing, flat plate assembly 14. Channel assembly 14 comprises an upper insulatorcontaining plate 15, a lower insulator-containing plate 16, an inner, insulating, sealing washer 17 and an outer, insulating, sealing washer 18. An aperture 19 in upper plate assembly leads to a reservoir 20 which is in contact with a first piston 21 which is held resiliently in place with a compressible member 22. A- threaded plug 23 holds piston 21 and resilient member 22 securely in place within header 11. Reservoir 20 contains an annular contact ring 24 of a metal such as copper, which wets the current carrying liquid metal well and serves as an arc footpoint. A second aperture 25 in lower plate assembly 16 leads to a current conductor reservoir 26 which is in contact with a piston 27 which is held resiliently in contact with metallic current carrying conductor 26 by a resilient member 28. An annular contact ring 24 is also contained in reservoir 26. Piston 27 and resilient member 28 are held securely in place by bolt 29 which threads securely into lower header 12. An entrance port to reservoir 20 is provided by bore 30 which is closed by setscrew 31. An entrance port to reservoir 26 is provided by bore 32 which is closed by setscrew 33. A filling port 34 and a threaded counterbore 35 leads to the threaded bore containing setscrew 33 and may be closed off thereby.

Although the device illustrated in cross-section in FIG- URE 1 may have any configuration, as for example, cylindrical, it has been found convenient to construct the device with a rectangular cross-section as is illustrated in FIGURE 2, which is a plan view of the device of FIGURE 1 and which is taken along lines 1-1 of FIGURE 2. In FIGURE 2 upper header 11, bolts 13, 31 and 23 are clearly visible.

FIGURE 3 of the drawing illustrates a perspective view of plate member 16. A vertical cross-sectional view of the same member is illustrated in FIGURE 4. In FIGURES 3 and 4 flat plate assembly 16 comprises a high ductility steel member 36 which is grooved to form a central, recessed region by machining a recessed surface 37 in one face thereof. The recessed surface 37 of disc 36 is then filled with a plasma-sprayed-in layer 38 of a high dielectric strength, insulating ceramic, as for example high density alumina. The alumina is then machined to form an outer annular ring 39, the exterior face of which is flush with the surface of steel disc 36, a recessed annular groove 40, which does not quite reach to the original surface 37 and a central mesa-like disc surface 41 which extends upward beyond the surface of disc 36. Surface 41 is then grooved by a slightly tapered slot 42, at one end of which is an aperture 25, which passes through the entire thickness of ceramic and metal, completely penetrating assembly 16.

The dimensions of the various machined portions assembly 16 vary in accordance with the voltage and currents to be utilized in connection with device 10. For example, however, in order that the invention may be completely understood, I have constructed one device suitable for operation at 220 volts and capable of interrupting fault currents up to 50,000 amperes and having dimensions as follows. The outer diameter of plate assembly 16 is 1.9"; the outer diameter of ceramic insert 38 is 1.772". The outer diameter of groove 40 is 1.526" and the inner diameter thereof is 1.406". The groove 42 is A" in length and in width and has a maximum depth of 0.006" tapering to a match with the surface of 41 at the ends thereof. Slot 25 has a dimension of Ms" thick and /3" long with the corners having a rounded fillet. The upper plate assembly 15 is identical with that of lower plate assembly 16 but, upon assembly, the aperture through assembly 16 aligns itself with the unapertured end of slot 42 as does slot 25 in assembly 16 align itself with the unapertured portion of the slot in the center of assembly 15. Slot 42 and its matching counterpart constitute the arcing chamber 60 of the current limiter device 10 and is the region in which the liquid current carrying metallic conductor first vaporizes to form a high resistance to the fault-current.

Headers 11 and 12 are counterbored in order to receive the flat plate assembly 14. This is accomplished by a large diameter bore being cut into the surface 50 of header 11 and the surface 51 of header 12. The first groove 53 has adiameter sufficient to accommodate the outer diameter of outer insulating annular washer 18. A lesser depth, lesser diameter bore 54 is cut in the central portion of the face 50 and 51 of headers 11 and 12 and has a diameter sufficient to accommodate the outer diameter of assemblies 15 and 16.

To assemble the limiter 10 as illustrated in FIGURE 1 of the drawing, the lower surface 55 of assembly 16 is coated with a thin layer 56 of still-fluid epoxy resin and the assembly 16 is placed firmly in position within bore 54 in header 12. An outer insulating washer which may for example be of fiberglass encased in solidified epoxy resin 18 is placed exterior of assembly 16. An inner, insulating washer 17, which may likewise be of fiberglass encased in solidified epoxy resin, is inserted in annular groove 40 of assembly 16. Compressible neoprene rubber washer 49 is placed about the exterior of fiberglass ring 17 and within fiberglass ring 18. Plate assembly 15 is also epoxy resin cemented in the bore within header '11 and the two headers are brought together so that the inner and outer fiberglass washers 17 and 18 mesh into the appropriate and correlative recesses in header 11 and plate assembly 15 as in header 12 and plate assembly 16. The slots 42 of each of plate assemblies 15 and 16 should automatically become aligned to form channel 60 as illustrated in FIGURE 1 of the drawing.

After assembly of the two headers together with the flat plate assembly 14, screws 13 are inserted in headers 11 and 12. Screws 13 abut against a metal washer 61 which abuts against an insulating washer 62 insulating bolt 13 from header 11. Similarly insulating sleeve 63 fits within the unthreaded hole within header 11 and likewise insulates bolt 13 from header 11. Bolts 13, however, are threaded into header 12 and are drawn up tightly to cause compression of neoprene rubber gasket 49 to place the same under sufiicient compressive strength as to withstand any initial shock, either thermal or mechanical, transmitted to device 10 upon a transition of the liquid current carrying conducting metal from liquid to vapor phase. Initially, the thickness of neoprene washer 49 is approximately 0.015" but under compressive stress applied by bolts 13 the thickness thereof is reduced to approximately 0.010" to cause a high compressive stress to be set up therein.

After the assembly of headers 11 and 12 together with flat plate disc assembly 14, copper rings 24, pistons 21 and 27, and resilient members 22 and 28 respectively are inserted in the reservoir bores and bolts 23 and 29 are securely fastened in headers and 11, respectively, to form the current carrying channel for the metallic current carrying conductor. The volume within the device comprising reservoirs 20 and 26 and channel 60 is filled with a metallic current carrying conductor 64, preferably a liquid which may for example be mercury, gallium, indium, or a mixture of mercury with a small percentage of one of the alkali metals such as cesium, sodium, potassium, lithium in an amount of approximately 10% or less, in order to decrease the minimum voltage drop which may be maintained within the arc during current limiting. To fill the device, in accord with the present invention, the setscrew 31 is inserted and firmly fastened in place and setscrew 33 is inserted but not firmly fastened so as to allow communication between reservoir 26 and filling port 35. A vacuum pump is connected to threaded filling point 35 and the channel is evacuated to a pressure of approximately 10- torr., after which mercury or another suitable conductive liquid, as mentioned hereinbefore, is admitted thereto under a pressure of from 1 to 5 atmospheres, and preferably at approximately 4 atmospheres. After the mercury has completely filled the reservoirs and the channel, setscrew 3'3 is tightened securely, thus closing filling port 35 and confining the liquid current carrying metallic conductor within the channel and reservoir volume.

FIGURE 5 illustrates an alternative embodiment to the structure of assemblies and 16. In FIGURE 5 a single alumina disc 70 has been machined so as to provide an annular peripheral region 71 of a given thickness, a flat mesa-shaped central region 72 into which a slotted groove 42 has been cut at the end of which an aperture 25 extends therethrough as in the assembly of FIGURE 3. Immediately exterior of mesa-like portion 72, a groove 40 is cut into the peripheral region 71 of member 70. Alumina member 70 then replaces assembly 16 in the device of FIGURE 1, thus taking the place of the steel plate with the sprayed alumina insert therein. The single piece has the same exterior dimensions as the composite assembly 16, and the member of FIGURE 5 is inserted in device 10 as assembly 16. Interior and exterior fiberglass seals 17 and 18, respectively, and neoprene rubber gasket 49 are assembled, prior to assembly of the upper and lower header assemblies and tightening of bolts 13, so that when bolts 13 are tightened, neoprene gasket 49 is compressed to set sufficient compressive force therein to withstand the mechanical and thermal shock.

The operation of the current limiting device in accord with the present invention is substantially as follows. During normal operation in an electric circuit, the device 10 of FIGURE 1 is connected in series with an electric current and with the breaker contacts of an associated current interrupting device, as for example a molded case circuit breaker. The operating coil of the breaker is connected in parallel circuit relationship with the limiter device and is responsive to an abrupt change in current thereof to interrupt the circuit. It is the function of current limiter 10 in accord with the present invention to cause the current which operates the breaker coil to be reduced so rapidly in magnitude that the breaker contacts are'not' subjected to the high magnitude fault-current. Accordingly, since the breaker coil response time may be a matter of 5 or 10 milliseconds, the operation of the limiter device must be accomplished in a matter of afew milliseconds. During normalcurrent operation, the current to an electrical load is carried through the liquid contained in the arc channel 60 and reservoirs and 26 of device 10.

Upon the occurrence of a fault in the line, a faultcurrent having a very rapidly rising characteristic, which may, for example, rise to a value of hundreds of thousands of amperes, begins to flow through limiter 10. When the fault-current reaches such a magnitude that the heating of the liquid current conductor raises the tempera ture thereof until the vapor pressure of the liquid metallic current conductor, as for example the mercury, reaches the fill pressure thereof. In this instance (assuming a fill pressure of 5 atmospheres of mercury), this point is reached at a temperature of approximately 750 C. When such ,a temperature is reached the metallic conductor begins to vaporize and bubbles form therein, eventually uniting to form a bubble which completely interrupts the current path through the channel 60. At that instant, the pressure within channel 60 rises to a very high value due to the tendency of the vapor to have a very much larger volume than in equal quantity of the liquid metallic conductor; The pressure of the vapor within channel 60 forces the liquid within the remainder of the conduction channel to recede and forces pistons 21 and 27 to compress compressible members 22 and 28 to cause an increase'of the total volume of the fluid reservoir in channel to be approximately as high as 5 times that of the volume prior to reciprocation of pistons 21 and 27. At that time suflicient mercury has receded from the reservoir to expose copper inserts 24 within reservoirs 20 and 26. Since copper is readily wetted by mercury, the arc footpoint tends to form upon the inner surface of inserts 24 and the arc continues to burn until extinction. Until the arc is extinguished, the energy contained therein is dissipated by erosion of the ceramic members which form the boundaries of arc-channel 60. While this dissipation is substantial, if maintained for any great length of time, it has been found that in devices in accord with the present invention the current is reduced to a very low value of a matter of hundreds of amperes Within a matter of a few milliseconds from the first occurrence of a fault. The relatively low current represented by that value is readily interrupted by the series connected, molded-case type circuit breaker which is energized in a matter of approximately 5 to 10 milliseconds. Accordingly, the erosion of the walls of channel 60 is not so great as to greatly shorten the lifetime of the device. Thus, limiter 10 may be repetitively used over and over again for limiting very high fault-currents to a value which may be interrupted by a relatively low power circuit breaker.

FIGURE 6 is a graph illustrating some of the impor- I tant parameters of the operation of the arc incurrent limiter devices in accord with the invention. It has been determined that the energy dissipated from the are burning within channel 60 during current limiting is dissipated through heat radiated to the relatively cool Walls of the channel and is governed by the relationship where E and I are the voltage gradient and current density, a the Stefan-Boltzmann constant, T the absolute temperature (which is assumed to be uniform throughout the channel) and s the thermal emissivity of the arc, and d is the hydraulic diameter of the channel. The hydraulic diameter is defined as four times the ratio of volume-tosurface of the arc channel. The hydraulic diameter of a long cylinder equals the cylinder diameter. The hydraulic diameter of a thin slab equals twice the slab thickness. The hydraulic diameter measures the average distance radiation must travel to escape from an optically transparent body of any shape. Thus, the exact configuration may be drawn from FIGURE 6 is that the gas within the arc behaves essentially, at temperatures above 10,000 K., as a fully ionized gas. In FIGURE 4, the ordinate represents the quantity E/\/e/d in vlts/cm. As abscissa I have plotted the relationship ]/'\/e/d in amperes/cm. Curve A in FIGURE 6 represents the resistivity characteristic of a fully ionized gas. Curves B, C, and D respectively represent the characteristics of mercury at 1, 10, and 100 atmospheres respectively. Dotted lines E, F, G, and H represent isothermal lines representing 2,000 K., 5,000 K., 10,000 K., and 20,000 K. respectively. Accordingly, it may be readily seen that the gases at approximately 1 to 10 atmospheres (the pressure within 1 which it has been determined the gases within the arc in devices in accord with the present invention operate) behave essentially as a fully ionized gas at temperatures above 10,000" K. It may further be determined that there are certain minimum voltage gradients below which the arc may not be sustained. It may readily be computed from the data containcd in FIGURE 6 and the equation cited hereinbefore, assuming a value of e/d of 1 cumthat it is necessary, utilizing a mercury arc, that the voltage gradient across the arc in device 10 be maintained at a value exceeding approximately 100 volts/cm. in order to sustain the are. On the other hand the voltage gradient must not be so high that excessive dissipation of the surface of the walls of channel 60 occurs. In accordance with the present invention, hydraulic diameters up to approximately 0.060" have been found to be suited for devices in accord with the present invention. Accordingly, in the device illustrated in FIGURE 1 of the drawing, a hydraulic diameter of 0.024" is chosen. Thus the depth of slot 42 in member 16 is chosen to be approximately 0.006. Accordingly, the channel should be sutficiently long and the voltage gradient should be as high as is consistent with the foregoing relationship to prevent destruction of the channel. Similarly, the length of the channel must not be so long that the cold resistance of the liquid mercury within the channel is too high and should not exceed approximately 1 milliohm.

Devices constructed in accord with the present invention have been successfully utilized to limit surge currents of as high as 50,000 amperes at 220 volts to a value of the order of several hundred amperes within approximately 3 milliseconds prior to interruption by a molded case type circuit breaker.

While the invention has been set forth herein with respect to certain specific embodiments thereof, many modifications and changes will readily occur to those skilled in the art. Accordingly, I intend by the appended claims to cover all such modifications and changes as fall within the true spirit and scope of the present invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A change-of-state current limiter device adapted to change from a first low-resistance condition in which current is carried therein by a liquid metal to a second high resistance condition in which current is carried therethrough in part by vapors of the same metal and adapted to return non-destructively to said first condition and comprising (a) means forming a channel in which said liquid metal is contained and in which a change-of-state thereof occurs in response to a fault-current and including a flat plate assembly comprising a pair of mating fiat plates having surface regions of high dielectric strength ceramic with mating grooves therein to form an arc channel,

(a1) each of said plates having an aperture therein at one end of said channel, said apertures being disposed upon assembly at opposite ends of said channel '1 (b) a pair of massive metallic headers abutting opposite exterior faces of said mated flat plates and including (bl) an aperture abutting the aperture in said mating flat plate at one end of said channel therein (b2) a metallic current conductor reservoir. in fluid contact with said aperture in said header (b3) arc sustaining means within said reservoir adapted to form a footpoint for a metallicyapor arc within said channel and said reservoirs 1.

(b4) piston means held resiliently in contact with a fluid metallic current carrying conductor within said reservoir,

(b5) compressible means behind said piston for allowing said piston to recede upon increased pressure within said reservoir and said channel to facilitate maintaining said pressure at as low a value as possible, and 1 (c) a quantity of a metallic current carrying conductor in the fluid state filling said channel and said reservoirs under pressure when said device isin an unactivated state. 1

2. The device of claim 1 wherein said flat plates are comprised of high density alumina.

3. The device of claim 1 wherein said flat plates are comprised of steel base members and a sprayed-upon layer of high density alumina.

4. The device of claim 1 wherein sealing betwecnsaid fiat plates is obtained by a plurality of insulating washers which are keyed into recesses in said flat plates.

5. The device of claim 4 wherein two of said insulating Washers enclose an annular deformable gasket member.

6. The device of claim 5 wherein said gasket is held in sufiicient compressive stress by means mechanically connecting said headers so as to decrease its volume substantially.

7. The device of claim 1 wherein said headers are electrically insulated and drawn together by mechanical force suflicient to set up within said flat plate assembly sufiicient compressive stresses as to resist thermal and mechanical shocks attendant a change of state of said metallic current conductor within said channel.

8. The device of claim 7 wherein said channel and said reservoirs are filled with mercury at a pressure of 1 to 5 atmospheres when not activated.

References Cited UNITED STATES PATENTS 2,306,728 12/1942 Heddaeus 338- XR 2,732,464 1/1956 Ohl 200-1 66 2,990,464 1/ 1961 Otterstedt 200-166 XR 3,016,436 1/1962 Lafferty 200-166 XR 3,117,203 1/ 1964 Hurtle 200-113 3,218,412 11/1965 Casey 200l13 BERNARD A. GILHEANY, Primary Examiner. H. B. GILSON, Assistant Examiner. 

